When considering understanding the contact state of a contact surface using a tactile sensor, there are vectors of three components representing magnitude and direction of force acting at each point of the contact surface. This is represented as f(x,y) in the coordinate system of FIG. 1. Here, f is a vector, and so actually has three components x, y and z at each point. When explicitly expressing each component, it is represented as f(x,y)=[fx(x,y), fy(x,y), fz(x,y)]. Since force distribution has three components at each contact point, in order to reconstruct force distribution for each contact surface using a tactile sensor, it is necessary to acquire information for each contact point on the contact surface with at least three degrees of freedom.
Some of inventors of the present invention et al. have proposed an optical tactile sensor that is capable of measuring three-dimensional force vector distribution. A principle of the optical tactile sensor will be explained based on FIG. 2. The optical tactile sensor comprises a transparent elastic body and a CCD camera. By photographing spherical markers 3, 4 embedded in the transparent elastic body by the CCD camera, internal strain information of the elastic body is measured when a force is applied on the surface of the elastic body, and force vector distribution is reconstructed from the information.
By taking an image of the spherical markers by a CCD camera from z-direction where an elastic body surface is taken as the x-y plane and an orthogonal direction to the x-y plane is taken as the z-axis, movement of a point to be measured when force is applied is measured as a movement vector in the x-y plane. However, it is difficult to reconstruct the force vector distribution from the strain information because an amount of information is insufficient. Therefore, N×N red spherical markers and blue spherical markers are arranged at different depths in the elastic body as points to be measured to obtain two sets of two-dimensional movement vectors with different depths as two pieces of different information, thereby increasing the amount of information to reconstruct the force vector distribution.
However, computation time required to obtain force vectors becomes long in proportion to four times the size (amount of information relating to behavior of markers) of the contact surface constituting the sensor surface. Thus, the computation time is longer with a larger contact surface, and there is a possibility of not being able to perform real time sensing. Also, even if the sensor surface has a relatively small surface area, if measurement density is high, the amount of information relating to behavior becomes high, time required for calculation of force vector distribution becomes long and it is not possible to perform real time sensing. Accordingly, when implementation and application of an optical tactile sensor are considered, shortening calculation time for obtaining force vector distribution is an important problem.
Patent Reference: WO02/18893A1
An object of the present invention is to provide a technique for reducing computation time for force vector calculation, even in the event that a sensor contact surface comprises a large surface area or measurement density of the contact surface is high.